Mustard greens — Food safety facts
Safe to share · Cats · full breakdown on the linked food page
Steamed and plain, small amounts.
The full verdict, portion guidance and serving notes are collected on the Mustard greens food facts page.
Short answer
Steamed plain mustard greens are safe for cats in small amounts, so no need to panic. Your first step is to check what else was with the greens — any butter, oil, garlic, or seasoning makes them unsafe. Next, just observe your cat for the next few hours; some cats might get a bit of gas or soft stool from new vegetables. Ensure fresh water is available. If your cat ate a large bowlful or shows vomiting, then a call to your vet is wise.